Studies are progressing with the PSA-1 embryonal carcinoma cell line which differentiates to cystic embryoid bodies under cell culture conditions. These studies involve a detailed analysis of DNA synthesis while the PSA-1 cultures are differentiating to embryoid bodies plus the selection of temperature sensitive growth mutants of PSA-1 embryonal carcinoma cells. We are currently using an effect described by others which allows for high cloning efficiencies of PSA-1 cells in the absence of feeder cells, the addition of mercaptoethanol to the growth media. This effect is being used in our mutant work.